In a data transmission system, if a fluctuation in the phase of a data signal to be transmitted, called a jitter, increases, the data signal may not be normally transferred. For this reason, it is necessary to measure the jitter characteristic of the data transmission system or equipment constructing the system in advance. In particular, in recent years, with higher-speed digital communication, many standards which define a jitter have been made as signal quality evaluation items.
A jitter means a fluctuation in the signal transition timing. A total jitter (TJ) representing a cumulative amount for one minute is divided broadly into a bounded deterministic jitter (DJ) in which a jitter occurrence range is determined and an unbounded random jitter (RJ) in which the jitter amount increases as the jitter is accumulated. The deterministic jitter is further classified into a periodic jitter (PJ), a data dependent jitter (DDJ), and a duty cycle jitter (DCD).
On the other hand, even when the data signal in the data transmission system has a voltage sufficient to identify Mark “1”/Space “0” communication errors may occur due to the occurrence of the jitter. In analyzing the cause for the occurrence of the jitter, there is a significant advantage in analysis by separating the jitter by feature (included in the standard). When measuring the jitters of two DUTs (Devices Under Test) to be measured only for a specific time, even if both jitter values are the same, the jitter value when the measurement time is extended differs between both DUTs due to a difference in the magnitude of randomness between the jitter values. For this reason, in the standard, such as Fiber-Channel, the jitters are classified and defined, and the measurement is defined.
On the other hand, as a method of measuring the jitter of the data signal in the data transmission system, there is a method in which a data signal is input to a waveform observing apparatus, the eye pattern of the data signal is displayed, and the jitter amount of the data signal is recognized from the width of an intersection portion of the rise and the fall of the displayed eye pattern. Patent Document 1 discloses a method which observes an eye pattern of a data signal to obtain a jitter.
However, in the method which measures the jitter on the basis of the width of the eye pattern displayed on the waveform observing apparatus, it is not possible to recognize a data dependent jitter (DDJ) which occurs depending on the pattern of the data signal.
In measuring the jitter of the data signal, while there are various measuring instruments which capture the timing of the data signal, currently when the jitter value has become more generally defined, jitter measurement using a sampling oscilloscope becomes mainstream. For this reason, the manufacturers of various measuring instruments realize a jitter measurement function as an additional function of an existing sampling oscilloscope, and contribute to jitter measurement.
In the method using this type of sampling oscilloscope, predictive calculation is performed from the waveform acquired based on waveform observation to realize jitter measurement. For this reason, it is very important to acquire data more quickly and efficiently.